1. Field of the Invention
The present invention relates to a spark plug used for the igniter of an internal combustion engine.
2. Description of the Related Art
Generally, a conventional type spark plug is provided with a central electrode and a parallel ground electrode. The central electrode is protruded downward from the lower end face of insulator. The parallel ground electrode is arranged opposite to the central electrode and one end of which is bonded to main metal shell for igniting fuel mixed gas by spark discharge in an air gap between the central electrode and the parallel ground electrode.
To enhance ignitability in the air gap, a spark plug provided with auxiliary ground electrodes opposite to the side face of a central electrode in addition to a parallel ground electrode opposite to the end face of the central electrode is proposed in Unexamined Japanese Patent Publication (kokai) No. Hei. 5-326107, U.S. Pat. No. 5,581,145 and EP 0 774 812A. These auxiliary ground electrodes are not provided to fly sparks in a gap between the auxiliary ground electrode and the central electrode. However, these auxiliary ground electrodes are provided to improve the distribution of electric fields between the parallel ground electrode and the central electrode by the existence of the auxiliary ground electrodes. Accordingly, ignitability is enhanced by flying a spark in the gap between the parallel ground electrode and the central electrode at lower discharge voltage. Therefore, in the structure of these spark plugs, the edge of the end face of the auxiliary ground electrode is not necessarily positioned in the vicinity of the lower end face of the insulator.
Further, in U.S. patent application Ser. No. 08/749,309 and EP 0 774,812 A, a spark plug provided with an auxiliary ground electrode in the vicinity of the lower end face of insulator in addition to a parallel ground electrode opposite to the end face of a central electrode is proposed.
However, there is a problem that both conventional type spark plug disclosed in Unexamined Japanese Patent Publication No. Hei5-326107 and U.S. Pat. No. 5,581,145 are weak in a so-called carbon fouling. At the time of regular operation in which an internal combustion engine is rotated at engine speed equal to or faster than predetermined engine speed at predetermined temperature, a leg portion which is a lower part of the insulator of the spark plug is suitably burned and surface temperature in the vicinity of the lower end face of the insulator inside a combustion chamber rises up to approximately 500.degree. C. Therefore, carbon which adheres to the surface of the insulator is burned and the surface of the insulator is kept clean. Therefore, no carbon fouling is caused. However, in the case of a low load in which the temperature of the internal combustion engine is extremely low and the engine speed is also low, the surface temperature of the insulator does not rise, carbon by the combustion adheres to the surface of the insulator and is accumulated to be a so-called carbon fouling state. When this further progresses, insulation between the central electrode and the ground electrode is deteriorated, spark discharge is disabled and an engine stall is caused.
As for the conventional type spark plug disclosed in U.S. patent application Ser. No. 08/749,309, relationship between distance (a main air gap or a semi-surface gap) from the parallel ground electrode or the auxiliary ground electrode to the central electrode and distance (a gap between a semi-surface and the insulator) from the end face of the auxiliary ground electrode to the side face of the insulator is not disclosed.
Further, in Unexamined Japanese Patent Publication (kokai) No. Sho. 59-71279, a semi-surface spark plug in which an ground electrode is arranged opposite to the side face of insulator is disclosed. In the above spark plug, as sparks fly along the surface of the insulator, carbon which adheres to the surface of the insulator is burned off and the problem of a carbon fouling is hardly caused. However, as sparks always fly along the surface of the insulator, the problem of so-called channeling that the surface of the insulator is damaged by sparks is caused. Therefore, there is a problem that the life of the spark plug is short.